The overall aim of this proposal is to understand how sodium transport, in particular transport by the sodium pump, is regulated in the proximal and distal nephron. In both regions the sodium and potassium dependent adenosine triphosphatase (NaK-ATPase), also known as the sodium pump, drives the active reabsorption of sodium. Regulation of renal NaK-ATPase is important for maintenance of fluid, electrolyte and metabolic homeostasis. NaK-ATPase is regulated by thyroid hormone in the proximal tubule, mineralocorticoids in the distal tubule (either directly or indirectly through intracellular Na+ and/or K+), and glucocorticoids in both sections. The most likely hypothesis is that both synthesis and degradation are subject to regulation. Specifically, the effects of the suspected regulators (thyroid hormone, glucocorticoids, aldosterone, and intracellular Na+ and K+) on transcription, translation, synthesis and degradation of the alpha catalytic subunit of the NaK-ATPase will be studied. Primary cultures of kidney proximal tubules will be used to study proximal nephron and Madin Darby canine kidney cells (MDCK) for distal nephron. Regulation of transcription will be studied with a cDNA to the alpha subunit, and regulation of synthesis and degradation will be studied by immunoprecipitation of labeled alpha subunit from cells fractionated into defined membrane populations by analytical cell fractionation.